1. Field of the Invention
The present invention relates generally to storage systems and to backup and recovery of data.
2. Description of Related Art
A conventional method for achieving backup and recovery of data in storage systems is to backup the data periodically (e.g., once a day) from the storage system to a backup media, such as magnetic tapes. In creating the backup data, a snapshot of a storage area (e.g., a point-in-time copy of a volume) is often used to obtain data with consistency. That is, the data to be backed up is read from a snapshot volume (or a mirror image volume that has been split from a primary volume), and copied to the backup media. Several methods to provide a snapshot of a storage area logically or physically are well known in the prior art for storage systems. When it is necessary for data to be restored, the data saved to the backup media is read from the backup media, and may be written to a new storage area (e.g., a new volume) in the same storage system, or to a different storage system, to enable access to the backed-up data.
However, with the above method, it is only possible to restore the image of the data at the particular point in time at which the snapshot was taken. Thus, restoring data from backup data taken only periodically may result in a loss of some amount of updates to the original data because there is a difference between the saved back up data and the data that is actually desired to be restored, such as the current data. Further, for example, if the latest backup data has incorrect or corrupted actual data, it is necessary to use older backup data, which results in even larger disparities in data correctness.
Recently, some storage systems have been configured with a capability of journaling data updates into a journal as the data updates occur, and then, when necessary, restoring data using the journal in a technology that is sometimes referred to as “continuous data protection” (CDP). In a storage system having this capability, all updates for a storage area are recorded as a journal, and the data at an arbitrary point in time can be restored by using the journal. In this journaling and restoring technique, point-in-time snapshots may also be used. That is, besides the journal, snapshots of the storage area may be maintained at predetermined intervals, and restoring the data at a desired arbitrary point in time is achieved by applying the data of the journal to one of the snapshots up to the specified arbitrary point in time at which restore is desired. This technology is disclosed in detail in US Patent Application Publication Number US2004/0268067A1, to K. Yamagami, filed Jun. 26, 2003, the disclosure of which is incorporated herein by reference.
In the system with journaling capability discussed above, a substantial amount of storage area (storage capacity) may be required to store the journal. Maintaining excessive storage capacity dedicated to storing the journal can increase the total cost of ownership for the system. Thus, a fundamental problem with current CDP technology is that maintaining large storage areas dedicated to the storing of journal data increases the total cost of ownership for such storage systems. Therefore there is a need for methods and an apparatus to automatically adjust and attempt to minimize the amount of storage area dedicated to storing of journals. US Patent Application Publication Number US2004/0162958A1, to Kano et al., filed Feb. 23, 2004, the disclosure of which is incorporated herein by reference, discloses automated online capacity expansion for storage devices. However, the prior art does not address or provide a solution to the problem discussed above.